Coordinate measuring machines are used for dimensional inspection of workpieces, such as machined or molded parts. A workpiece typically is secured to a fixed table and a measuring probe is secured to a ram which is movable in three dimensions. To measure the position of a point on the workpiece, the probe is brought into contact with the point, and measuring scales or other sensors on the machine are read. The position of the point is typically expressed in X, Y and Z coordinates within a working volume of the machine. To measure a distance between two points, the points are contacted successively, the coordinates of both points are read, and the distance is calculated from the coordinates. State of the art coordinate measuring machines typically have features such as high resolution measuring systems, electrical contact probes, motor drives, computer controlled drives and computer acquisition and processing of data.
Two common types of coordinate measuring machines are a moving bridge machine, and a gantry style machine. In both, a bridge moves in the Y direction along guideways on a table or support. A carriage moves in the X direction along guideways on the bridge. A ram with a probe mounted on its lower end moves vertically or in the Z direction through bearings in the carriage. Thus, three-dimensional movement of the probe is provided. The scales associated with each of the movable elements indicate the positions of the movable elements in the three axial directions.
The accuracy of a coordinate measuring machine is limited by geometric errors, such as inaccuracies in the scales or other measuring devices, and faults in the guideways or other elements which define machine motion. These inaccuracies can cause measurement errors when the machine is operated at a reference temperature, which is usually 20° C. One known approach to increasing accuracy with respect to the geometric errors is to improve the construction techniques and to reduce tolerances of the system so that errors are reduced. However, reduction of errors becomes progressively more expensive as required accuracies increase. Another known approach is direct measurement of coordinate errors at points throughout the machine working volume. This approach is impractical because of a huge amount of data which must be stored for large machines and because of the time required to measure such data. In one example, U.S. Pat. No. 4,884,348 discloses a testing device for determining measurement errors.
A third known approach is the measurement of errors in parametric form. As noted, a coordinate machine typically has three sets of guideways which establish probe motion. Ideally, movement along each of these guideways should result only in linear motion, and a scale reading should equal the linear displacement. In reality, however, there are scale errors and the guideways are not completely straight or perfectly free from twists. For such a machine, there are six degrees of freedom which produce errors during movement along each guideway. For each direction of movement, there are three linear errors, DX, DY and DZ and three rotational errors AX, AY and AZ. The six error parameter is measured at a number of points along each direction of machine movement, resulting in an error matrix with 18 error parameters. From the matrix of 18 error parameters, the error at any point in the measurement volume is calculated and stored. The calculated errors are then subtracted from the measured coordinate values to determine actual workpiece coordinates. Examples of this approach are found in U.S. Pat. Nos. 4,884,889 and 4,939,678.
In addition to the above mentioned so-called geometric errors, the accuracy of a coordinate measuring machine is in general also affected by thermally induced errors that may cause deformation of machine components. These are errors resulting from thermal expansion, or differential thermal expansion, due to differing coefficients of thermal expansion of different machine components. It is well known that these measurement errors may be minimized by maintaining a coordinate measuring machine at a constant temperature to prevent changes in size of the various components due to thermal expansion or contraction. However, it is not always possible to maintain the environment surrounding a coordinate measuring machine at a constant temperature. This is particularly true in a shop environment where temperatures and humidity conditions will change from season to season and day to day.
In a manner that is analogous to the correction of geometric errors, the thermal errors can be minimized either by applying appropriate design techniques or by using software error compensation techniques. The latter method is based on taking real-time readings from temperature sensors mounted on the measuring machine and the use of a model that resides inside the machine's controller. The model relates the sensor readings to geometric deformations caused by temperature changes. Correction values for the scale readings are calculated to offset these errors.
In the former method, various construction techniques may be used, such as employing materials with low coefficients of thermal expansion, or materials which all have the same coefficient of thermal expansion. Examples of such construction techniques are found in the following patents and applications: U.S. Pat. Nos. 4,538,911; 5,173,613; 5,198,874; 4,962,591; and 5,031,331; and Publication Nos. WO 89/09920 and WO 89/09887.
It is not always practical to make all components of the same material, or materials having the same coefficients of thermal expansion, since each component serves a different function and therefore should have properties that are different from other components. For example, it is desirable that the bridge be strong, but not necessarily heavy so that the bridge has a relatively low inertia. On the other hand, the guideways upon which the bridge and ram travel must be strong and formed of a material such as steel which provides a precision pathway. If the bridge were made of steel like the guideways, it would be too heavy to be of practical use and likely would be too expensive.
Most known prior art systems for fully temperature compensating a coordinate measuring machine are relatively complex and expensive. Therefore, it is desirable to have a coordinate measuring machine which can be used in various temperature environments and formed of materials of different coefficients of thermal expansion, and yet still possess a very high level of accuracy without the need for an expensive thermal compensation system.